The present invention relates to a liquid ejecting head which ejects liquid droplets from nozzle openings by vibration of pressure generating elements.
The liquid ejecting head provided with the pressure generating elements is known in those of dealing with various kinds of liquids, and among of them, typical may be a recoding head employed in an ink jet-recording apparatus. Therefore a recording head of the ink jet-recording apparatus as one example of the related liquid ejecting head will be described according to FIGS. 8 and 9 of the attached drawings.
The recording head includes a passage unit 1 having nozzle openings 2 and a head case 9 to be attached with the passage unit 1.
The passage unit 1 is composed by laminating a nozzle plate 3 provided in row with the nozzle openings 2 in a nozzle forming face 3A, a passage substrate 5 provided in row with pressure generating chambers 4 communicating with the respective nozzle openings 2, and a vibrating plate 6 for closing the lower openings of the pressure generating chambers 4. The passage substrate 5 is formed with ink reservoirs 8 which communicate with the respective pressure generating chambers 4 via ink passages 7, and hold an ink to be flowed to the pressure generating chambers 4. incidentally, the whole of the recording head is shown with a mark of H.
The head case 9 served as a base material of the recording head H is formed by injection molding of a thermosetting resin or a thermoplastic resin, and has spaces being vertically extending. The spaces accommodate pressure generating elements 11. The pressure generating elements 11 are fixed to fixing substrates 12 by lower ends thereof being attached to the head case 9. And front end faces of the pressure generating elements is fixed to island portions 6A at an under face of the vibrating plate 6.
A plurality of pressure generating chambers 4, pressure generating elements 11, and nozzle openings 2 are arranged in a perpendicular direction with respect to the sheet surface of FIG. 9. That is, in this example, a nozzle arrangement is formed in two rows, such that the same kind of ink is ejected from each row of the nozzles as one unit.
The pressure generating elements 11 are connected to conducting wires 13 for input respectively, as shown in FIGS. 8 and 9. Each conducting wire 13 is connected to a printed wiring 15 on the head substrate 14 through each through hole 14A of the head substrate 14. The printed wiring 15 is made intensive and connected to a flexible flat cable 17 through a connector 16. The flexible flat cable 17 is connected to a drive circuit (not shown). When a drive signal from the drive circuit is input to the pressure generating elements 11, the pressure generating elements 11 are caused to be expanded and contracted in longitudinal directions to change pressure in the pressure generating chambers 4, whereby the ink in the pressure generating chambers 4 is ejected as ink drops from the nozzle openings 2. Incidentally, the through holes 14A are filled with the inserted conducting wires 13 so that the through holes has almost no spaces, but for easily seeing air flowing condition thereabout, dimensions of the through holes 14A are illustrated by exaggeration.
On the other hand, damper recesses 18 for escaping pressure fluctuation in the ink reservoirs 8 in ejecting the ink through the vibrating plate 6 is formed at parts of the head case 9 corresponding to the ink reservoirs 8. The vibrating plate 6 is made of polyphenylene sulfide film (called as “PPS film” hereafter). When the damper recesses 18 are formed as independent spaces being not communicating with an outside, the air in the damper recesses 18 penetrates through the vibrating plate 6 made of PPS film and emerges into the ink, so that an air pressure in the damper recesses 18 is dropped, and tension of the vibrating plate 6 is rose, and sufficient damper effect cannot be provided. Therefore, outside communication passages 19 are bored directing from the bottom of the damper recess 18 toward an opposite side of the head case 9 for the damper recesses 18 to communicate with the outside in order to prevent reduction of the pressure in the damper recess 18.
However, since an opening area of the damper recess 18 is large, an area of the vibrating plate 6 covering the opening area is also large accordingly, and in particular, while the ink jet-recording apparatus is at rest, a water content in the ink is evaporated and passes through the vibrating plate 6 having the wide area into the damper recesses 18, and the vapor is released in an atmosphere via the outside-communication passages 19 in accordance with increase of the pressure. Owing to such phenomenon, the water content in the ink decreases and viscosity of the ink increases, and when the apparatus is used after some later, an obstacle occurs in ejection of proper ink drops.
Then, for decreasing evaporation of the water content in the ink as fast as possible, the outside-communication passages 19 are formed with parts of small flowing area, otherwise passage shapes are formed with bent parts having large flowing resistance so as to restrain the water evaporation while functioning the damper.
In the above related apparatus, some measures are taken to the parts of the damper recesses 18. However, as shown in FIG. 9B, the vapor passes the vibrating plate 6 effected with displacement in vibration of the pressure generating elements 11, that is, passes vibration displacing parts 6B of the vibrating plate 6 positioning around island portions 6A, is released from the spaces 10 into the atmosphere through slight gaps created in face-contacting parts between the head substrate 14 and the head case 9 or through the through holes 14A. Accordingly, for decreasing evaporation of the water content in the ink as fast as possible, it is necessary to suppress the vapor passing the vibration displacing parts 6B from releasing into the atmosphere.
The vapor of the liquid passing through the vibration displacing parts 6B is a vapor of the liquid itself, or a vapor of specific components in the liquid. Even if the vapor is any one of both, it spoils the liquid or hinders maintenance of normal composition of the liquid.